Obtaining spectral information from a moving object

Abstract

An optical device includes a waveplate sandwiched between first and second polarizers and is arranged to receive light emanating from an object or object image that is in motion relative to the optical device. A detector array includes one or more detector elements and is optically coupled to receive light from the second polarizer. Each detector element of the detector array provides an electrical output signal that varies according to intensity of the light received from the second polarizer. The intensity of the light is a function of relative motion of the object or the object image and the optical device and contains spectral information about an object point of the object.

The invention claimed is: 
     
       1. A method, comprising;
 receiving emanating light from at least one point (object/image point) of an object or image of the object as the object/image point moves along a trajectory relative to a waveplate of a spectral encoder , the trajectory having a length, L, within a field of view of the spectral encoder, the object/image point having a diameter, d, along the trajectory; 
 for the at least one object/image point:
 simultaneously detecting positional information and obtaining spectral information for the object/image point comprising:
 generating a position-dependent polarization interferogram from the emanating light, the position-dependent polarization interferogram comprising N interference fringes generated as a function of time from light at wavelength λ emanating from the object/image point as the object/image point traverses the length L, wherein L≥2dN; 
 converting the position-dependent polarization interferogram into a time varying electrical output signal; and 
 determining spectral information corresponding to the object/image point, the spectral information determined from the time varying electrical output signal and obtained from the position-dependent polarization interferogram, the spectral information having a spectral resolution at wavelength λ of about λ/N. 
 
 
 
     
     
       2. The method of  claim 1 , further comprising compensating for angular sensitivity of the waveplate. 
     
     
       3. The method of  claim 1 , further comprising at least one of:
 imaging light from the object or the object image moving relative to the optical device onto the waveplate; and 
 imaging light from the waveplate onto the detector array. 
 
     
     
       4. The method of  claim 1 , wherein a zero retardance point of the waveplate is disposed at a specified position along the trajectory. 
     
     
       5. The method of  claim 1 , wherein a fringe localization plane of the waveplate coincides with a surface of a detector that converts the position-dependent polarization interferogram into the time varying electrical signal. 
     
     
       6. The method of  claim 1 , wherein determining the spectral information comprises performing a Fourier transformation of the time varying electrical output signal and determining the spectral information from the Fourier transformation. 
     
     
       7. The method of  claim 1 , wherein simultaneously detecting positional information and obtaining spectral information for the object/image point comprises:
 detecting encoded spectral information corresponding to the positional information over a time duration of the relative motion between the object/image point and the spectral encoder; 
 acquiring the spectral information of the object/image point over the time duration of the relative motion; and 
 providing hyperspectral image data having one or more positional axes and a wavelength axis. 
 
     
     
       8. The method of  claim 1 , further comprising correcting for at least one of optical dispersion in the waveplate and angle of incidence of light received by the waveplate. 
     
     
       9. A system, comprising;
 a spectral encoder comprising a waveplate disposed between first and second polarizers, the spectral encoder configured to receive emanating light from at least one point of an object or image of the object (object/image point) as the object/image point moves relative to the waveplate of the spectral encoder along a trajectory of length L within a field of view of the spectral encoder, the object/image point having a diameter, d, along the trajectory; 
 circuitry comprising:
 a detector configured to:
 simultaneously detect positional information and obtain spectral information for the object/image point from the spectral encoder; and 
 convert a position-dependent polarization interferogram generated by the spectral encoder from the emanating light into a time varying electrical signal, the position-dependent polarization interferogram comprising N interference fringes generated as a function of time from light at wavelengthλ emanating from the object/image point as the object/image point traverses the length L, wherein L≥2dN; and 
 
 a processor configured to determine spectral information corresponding to the object/image point from the time varying electrical signal at a spectral resolution at wavaelength λ of about λ/N. 
 
 
     
     
       10. The system of  claim 9 , further comprising at least one of:
 a lens configured to image light from the object or the object image moving relative to the optical device onto the waveplate; and 
 a lens configured to image light from the waveplate onto the detector. 
 
     
     
       11. The system of  claim 9 , wherein the detector and the waveplate are arranged such that there is a fixed correspondence between each detector element and at least one retardance of the waveplate. 
     
     
       12. The system of  claim 9 , further comprising a birefringent layer disposed between the object/image point and the detector, the birefringent layer having a substantially constant thickness and an optical axis arranged along an imaging axis of the spectral encoder. 
     
     
       13. The system of  claim 12 , wherein:
 the waveplate is a Wollaston prism comprising first and second halves; and 
 a thickness of the birefringent layer is equal to one half a thickness of the Wollaston prism. 
 
     
     
       14. The system of  claim 9 , wherein:
 the waveplate comprises a Wollaston prism comprising first and second prism halves, each of the first and second prism halves having a first thickness, t 1 , along a thinnest portion of the wedge and a second thickness, t 2 , along a thickest portion of the wedge and t 1  is less than 75% of t 2 . 
 
     
     
       15. The system of  claim 9 , wherein:
 the waveplate is a Wollaston prism; and 
 further comprising a first layer and a second layer, birefringence of the first layer being substantially equal in magnitude and opposite in sign with respect to birefringence of a first Wollaston prism half and birefringence of the second layer being substantially equal in magnitude and opposite in sign to birefringence of a second Wollaston prism half, wherein thicknesses of the first layer and second layer are substantially equal to average thicknesses of the first half and second half of the Wollaston prism, respectively. 
 
     
     
       16. The system of  claim 9 , wherein a fringe localization plane of the waveplate coincides with a surface of the detector. 
     
     
       17. The system of  claim 9 , wherein:
 there is a predetermined correspondence between each detector element of the detector and a position on the waveplate having a particular optical retardance; and 
 the processor circuitry is configured to determine wavelength information about the object based on the predetermined correspondence. 
 
     
     
       18. The system of  claim 17 , further comprising a mirror array having multiple movable mirrors wherein the mirror array is configured to provide the predetermined correspondence. 
     
     
       19. The system of  claim 9 , further comprising a movement mechanism configured to move the object or object image relative to the optical device. 
     
     
       20. The system of  claim 19 , wherein the movement mechanism is a vehicle.